I spent time looking at this yesterday.
Got it all set up and did some trial measurements but not properly as there were gardeners cutting grass!
The latest and EXCELLENT REW101 pdf outlines ASIO4ALL and I am sure this is the right way (only way?) to use REW with HDMI out and the Emotiva (or other) USB mic
You can select different HDMI channel outputs.
1 is FL
2 is FR
3 is centre
4 is subwoofer
etc
If you output to FL then the RMC-1 will deal with the FL channel as per your 1) Speaker config and 2)DSP settings. This may be why you are getting strange signals
2) You want all DSP off - No loudness, no Dynamic Range Control, No equalisation active
1) If you have FL set to small, with 60Hz crossover the the signal output from REW will start in your subwoofer, then the RMC-1 will fade it into the FL as per your RMC-1 crossover at 60Hz
Surround modes like Direct will bypass the crossover and the sub will get no signal unless you send to it directly. I don't think this is the right way to do it in REW, but I am hoping someone else will chime in.
I had been thinking a 0-200Hz sweep directly to the sub (HDMI channel 4) with no crossover on the target line in REW may be best.
I have some parametric (but Fixed frequency and Q) filters built in to my sub, so adjust those to get the trace better, and leave less work for Dirac (when it comes)?